Not Applicable.
Not Applicable.
1. Field of Invention
This invention relates to an apparatus for measuring the power consumption of an electricity powered device or machine. More specifically, this invention relates to an apparatus which measures the power consumption by calculating the wattage consumed by the electrical device from the measured voltage and current.
2. Description of the Related Art
The power consumed by any process is an important parameter useful for optimizing a process. For example, in grinding, the power consumed is indicative of the efficiency of a grinding wheel to remove material in a given operation. Thus, between two grinding wheels removing material at a certain rate under identical grinding conditions, the grinding wheel consuming less power is more efficient. Accordingly, it is desirable to measure the power consumed by a given machine.
Efficiency may also be calculated based upon the current draw of the load; however, the current draw is typically nonlinear for an inductive motor load wherein the power is a multiple of the voltage, the current, and a xe2x80x9cpower factorxe2x80x9d which is the cosine of the lag angle between the voltage and the current. The power factor varies depending upon the load. For example, an AC motor at idle may have a typical power factor of approximately 0.1 while the power factor of same motor at full load may be approximately 0.8. Measurement of the power consumed by an electrical load containing resistance, capacitance, and inductance requires the measurement of the actual values of the voltage, the current, and the phase angle representing the lag angle between the voltage and the current.
Other devices have been developed to calculate the power consumption by measuring the voltage and the current. Typical are those devices disclosed in the following United States Patents:
Typically in the inventions disclosed in the aforementioned patents, the power consumption is calculated by the Hall device which senses the magnetic field created in a current carrying line by the Hall Effect and multiples it by the line voltage, e.g., U.S. Pat. No. 5,438,258, issued to Maruyama and U.S. Pat. No. 4,764,720 issued to Nystrom. The prior art sensors based on the Hall device are essentially limited to lower frequency applications. The power calculated using a typical Hall device is limited by the frequency response of such a device and typically loses significant accuracy at frequencies above 1000 Hz. U.S. Pat. No. 5,537,029, issued to Hemminger et al., teaches a sophisticated microprocessor-based system having elaborate software for monitoring power consumption and, more important, testing the operation of the power meter. The present invention is capable of accurate measurements at frequencies above the 1000 Hz mark by using a Hall sensor only to obtain a voltage proportional to the current and independently obtaining and attenuating the input voltage. The input-current voltage and the input voltage are multiplied and summed to calculate the total power consumption using simple, discrete components.
Additionally, the prior art devices typically require a complex setup procedure requiring the operator to select both the maximum input current and the maximum input voltage. The present invention reduces the need for additional settings requiring the operator to simply select the desired output range of the power meter.
Finally, the prior art devices are limited in the customization and information available to the operator. The devices are typically intended for self-contained use. For example, when using a portable power meter which is not associated with a single machine, it is desirable to be able to adjust the scale of the output to read the power consumption with some precision. Further, prior art devices typically provide instantaneous power consumption readings using an analog display which suffers from a slow response time and hides small effects from the operator. Additionally, the prior art meters, particularly those with analog displays, do not provide the operator with important information such as peak power. Prior art devices incorporating digital displays have not been taught having differing measurement units available to the operator. While the analog displays may have multiple scales, the analog devices typically only provide an approximation of the total power consumption subject to the operator""s interpretation of the needle position. The present invention provides an unscaled output along with an operator selectable scaled output allowing more precise measurement of power consumption significantly less than the full scale maximum of the power meter in addition to a default range, full scale output. Additionally, the present invention utilizes a bar LED display for rapid response when displaying the instantaneous power usage and a digital display with user selectable measurement units to provide accurate, non-discretionary power consumption readings coupled with a peak power hold feature. Finally, the power meter of the present invention, while self contained, provides for intelligent communication with external devices.
Accordingly, there is a need for a power meter which does not require setting of the maximum input current and the maximum input voltage prior to making a measurement. Additionally, there is a need for a power meter which is capable of measuring the power consumption of a load requiring a high frequency power supply without degradation of the measurement accuracy. There is also a need for a power meter providing a fast responding display of instantaneous power consumption, a display of peak power consumption, and providing operator selectable measurement units. Further, there is a need for a power meter providing both scaled and unscaled outputs for external monitoring of the system and providing both high and low trip points. Finally, there is a need for a power meter capable of communicating with and responding to external devices.
Therefore, it is an object of the present invention to provide a power meter which requires only the maximum power range to be set prior to making a measurement.
It is another object of the present invention to provide a power meter which is capable of measuring the power consumption of a load requiring a high frequency power supply without degradation of the measurement accuracy.
Yet another object of the present invention is to provide a power meter which provides a fast responding display of the instantaneous power consumption of the load.
A further object of the present invention is to provide a power meter which communicates with external devices for remote control of the power meter and remote display of the power meter settings and outputs.
A still further object of the present invention is to provide a power meter which simultaneously provides scaled and unscaled output signals.
An additional object of the present invention is to provide a power meter which can store and display the peak power consumption of the load.
Another object of the present invention is to provide a power meter having both high and low trip points in a single unit.
Yet another object of the present invention is to provide a power meter which is capable of scaling the output to a unit of measure selected by the operator.
An apparatus for measuring electrical power consumption is provided. The power is measured by multiplying the instantaneous voltage of the load by the instantaneous current of the load. The current is measured using a Hall Effect current sensor. The output of the Hall current sensor is a voltage signal proportional to the measured current which is amplified to a useful level by an input current amplifier. Additionally, the input current amplifier includes a variable resistance which can be adjusted to compensate for the offset voltages added by the Hall Effect current sensor and the input current amplifier itself. A scaled down input voltage representing the instantaneous voltage of the load is obtained at an input voltage attenuation. The attenuation includes a protection circuit which protects the power meter from damage in the event that the input voltage exceeds the rated input voltage of the power meter.
The corresponding phases of input voltage and the input-current voltage are multiplied and summed to calculate the instantaneous power. The input-current voltage and the attenuated phase voltage are applied to an input of an analog multiplier. The product of the two voltages is proportional to the instantaneous power in that phase. When multiple phases are involved, the output voltages of the multipliers are summed to produce a voltage representing the total instantaneous power for all phases. A calibration amplifier adjusts the output voltage of the adder such that the output voltage is ten volts (10 V) at full scale. Next, a precision isolation amplifier isolates the adder output from the display and output portion of the power meter from the high power portion of the power meter. The output voltage of the isolation amplifier represents the total instantaneous power consumption of the attached load.
The total instantaneous power consumption voltage, or total power voltage, is used to provide the operator with information regarding the power usage of the load. The total power voltage is calibrated to the full scale of the power meter for display to the operator. The power meter provides the operator the ability to select the measurement units in which the digital representation of the total power voltage is displayed. If desired, the idle power can be subtracted from the total power. The power meter also stores and displays the peak value of the digital representation of the total power at the operator""s request. A trip point detector provides monitoring of both high and low adjustable trip points along with an operator selectable time delay requiring the power to remain above or below the predetermined threshold for a predetermined period of time to prevent false triggers resulting from noise. The trip point detector includes a trip output for connecting an external warning or control device.
The output of the isolation amplifier also drives a scaling amplifier for enhancing the sensor range. The gain of the scaling amplifier is controlled by the operator adjustable range selector. The actual range is equal to the power meter full scale range divided by range factor selected using the range selector thereby allowing measurement of power at less than full scale with the desired degree of precision. The range-adjusted power is instantaneously displayed as a percentage of the range scale selected by the operator, in increments determined by the resolution of the selected LED bar display.
Various external outputs are provided to allow the power meter to interface with external devices, such as for control applications. The power meter includes both an unscaled and a scaled external voltage output and a current output for control applications. Finally, the power meter can be monitored and controlled from an external device.
An apparatus for measuring electrical power consumption has been disclosed having advantages over the prior art. The present invention does not require setting of the maximum input current and the maximum input voltage prior to making a measurement. Additionally, the present invention is capable of measuring the power consumption of a load requiring a high frequency power supply without degradation of the measurement accuracy. The power meter of the present invention provides a display of instantaneous power consumption as a percentage of the scaled measurement range. Further, the present invention provides a display of peak power consumption and operator selectable measurement units. The present invention also simultaneously provides both scaled and unscaled outputs, along with both high and low trip points. Finally, the power meter of the present invention is capable of communicating with and responding to external devices.